1. Field of the Invention
The present invention relates to a control system and method for an internal combustion engine that is operated while switching a combustion mode between a compression ignition combustion mode in which a mixture supplied to a combustion chamber is burned by compression ignition, and a spark ignition combustion mode in which the mixture is burned by spark ignition.
2. Description of the Related Art
Conventionally, there has been proposed a control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-257467, in which an operating state of an internal combustion engine is determined based on the rotational speed of the engine and demanded torque, and if it is determined the engine is in a low-to-medium load operating state, combustion in a compression ignition combustion mode is executed, whereas if the engine in the other operating state, combustion in a spark ignition combustion mode is executed.
The higher the temperature of operating gases before the start of the compression stroke, including an air-fuel mixture and EGR gases present in the combustion chamber, the earlier the timing of combustion ignition in the compression ignition combustion. However, in the conventional control system, insofar as the engine is in a low-to-medium load operating state, the compression ignition combustion mode is selected, and hence if the temperature of operating gases is too low during the compression ignition combustion mode, the timing of compression ignition becomes so late that the combustion becomes unstable, which can cause a misfire. Inversely, if the temperature of operating gases is too high, ignition tends to become so early that knocking can occur.